1. Field of the Invention
The present invention relates to bus line drivers, and more particularly, to a driver which meets the specifications for the Universal Serial Bus and that does not utilize linear poly-capacitors.
2. Description of the Related Art
Universal Serial Bus (USB) is a new industry standard for a cable bus that supports data exchange between a host computer and a wide range of simultaneously accessible peripherals. It provides for connection of a personal computer to a telephone line and can be used for port expansion.
The USB standard specifies a differential output driver to drive the data signal onto the USB cable at a data rate of 12 MBS for full speed and 1.5 MBS for low speed. Some of the electrical specifications for the USB standard are sown in Table I. The USB specifications provide challenges to the circuit implementation, especially in CMOS area.
TABLE I ______________________________________ Full Speed (FS) Low Speed (LS) at C.sub.L at C.sub.L = 50 to 350 pf Transition MIN MAX MIN MAX ______________________________________ Rise Time (TR) 4 n 20 n 75 n 300 n Fall Time (TF) 4 n 20 n 75 n 300 n TR/TF Matching 90% 110% 80% 120% Diff. Signal 1.3 V 2.0 V 1.3 V 2.0 V Crossing PT ______________________________________
The output voltage swings between the two single-ended outputs of a USB driver should be complementary. Furthermore, the output swings between differential high and low should be well balanced, i.e., .+-.10% matching on a signal transition of 12 ns for full speed and 120 ns for low speed, and have a controlled slew rate in order to minimize signal skew, the radiated noise, cross talk and common mode reflection.
In the low speed switching mode, matching the rise time TR and the fall time TF, i.e., designing the driver so that they are within 20% of each other, is challenging in view of the variations in the load capacitance of the cable. Specifically, a long cable creates a high load capacitance and a short cable creates a low load capacitance, causing the load capacitance in the low speed switching mode to change from 50 pF to 350 pF. The difficulty in matching the rise time TR and the fall time TF is in part due to the high load capacitance of the long cable causing a slow rise time TR, and the low load capacitance of the short cable causing a fast rise time TR.
Thus, there is a need for a USB driver which can closely match the rise time TR and the fall time TF over variations in load capacitance.